Unconventional sources of peptides for antigen presentation Project Abstract The long term goals of this project are to understand how the antigen processing pathway generates peptide/MHC class I complexes (pMHC I) by cryptic translation. Numerous studies, from different laboratories, have shown that naturally processed pMHC I on the surface of tumors, virally infected cells, transfected or even normal cells arise from regions of mRNAs that were not expected to be translated. The mechanism that allows cells to sample this cryptic source of antigenic precursors versus other conventional sources are poorly understood. Moreover, the conditions that enhance presentation of cryptic peptides in cells are not well defined. Here we will test the hypotheses that (a) cryptic pMHC arise due to translation initiated by a distinct and novel set of tRNAs and ribosomes, (b) expression of cryptic pMHC I is regulated in cells under stress, and (c) the eIF2A translation initiation factor makes a key contribution to the cryptic pMHC I repertoire and immunity. We anticipate that an improved understanding of how cryptic pMHC I are generated will not only provide a means to tap into a novel sources of antigenic peptides and new approaches to vaccine design for intractable viruses and cancer, but will also yield new insights into protein translation and immune surveillance mechanisms.